Phosphate conversion coatings are well known for treating metal surfaces, particularly ferrous, zinc and aluminum metals and their alloys. When applied, these phosphate coatings form a phosphate layer, primarily of zinc and iron phosphate crystals, which provides corrosion resistance and/or enhances the adhesion of subsequently applied coatings.
Prior to application of the phosphate coating, the metal substrate is typically “conditioned” or “activated” by subjecting the surface of the metal substrate to a diluted aqueous dispersion, sometimes referred to as an activating rinse or activator, by introducing or immersing the metal substrate into a tank that contains the activating rinse bath. “Activation” of the surface of the metal substrate is achieved due to the adsorption of colloidal titanium-phosphate particles, which are present in the activating rinse bath, to the metal's surface. These colloidal titanium-phosphate particles, however, have a tendency to agglomerate in the activating rinse bath due to dissolved calcium (Ca2+) and magnesium (Mg2+) ions (hard water ions) that are typically present in the rinse conditioner bath.
Zinc phosphate and other divalent metal phosphate dispersions also may be used to “activate” the surface of the metal substrate, but such dispersions are generally unstable.